Re: Peculiarities in demo plots

[Daniel J S. <dan...@ie...>]

On 04/20/2015 12:02 AM, sfeam wrote:
> On Sunday, 19 April 2015 11:51:22 PM Daniel J Sebald wrote:
>> Forgot one:
>>
>> imageNaN.dem
>>     Treatment of missing/undefined/NaN/Inf data: Qt and x11 plots look
>> similar except for first column.
>>                Qt: (top to bottom) black, yellow, white, blue, white, blue
>>                x11: (top to bottom) black, yellow, black, blue, black, blue
>>     image from non-matrix data: The x11 plot has a black pixel left,
>> second from top so the text "NaN should appear as background" is not
>> visible.
>>     negative values mapped to log-scale colorbar: Same sort of thing for
>> x11, the "Negative values become NaN..." is not visible.
>
> Right. That's the point of having the demos - they serve as unit tests
> for the various code features.  In this case it shows that the x11 terminal
> does not handle NaN in image data optimally.  That's at least partly because
> the x11 terminal doesn't handle transparency, so there is not the option
> of mapping NaN to "all-transparent".
>
> The PostScript terminal exhibits the same deficiency.
> In both cases the terminal is just showing it's age.
> The newer options (qt/cairo) are more capable.
>
> 	Ethan

Are we talking 100% transparent "pixel"?  Or partially transparent?

I've not worked with any of the Qt QGraphicsItem classes, but I'm 
wondering if this QGraphicsPixmapItem class is handling images as a 
collective of pixels, as opposed to an image of data that is sent to a 
driver.  That might explain the slowness of the Qt terminal.  Qt is much 
slower than x11 when working with the image data--a factor of five or more.

In other words, the QGraphicsItem class might be similar to the failsafe 
mode of x11, as far as its operation is concerned--I'm assuming failsafe 
is where an image is created by drawing squares on the canvas.

For that example, if I turn on failsafe mode, i.e.,

plot $DATA with image failsafe

that example with "NaN should appear as background" and "Negative values 
become NaN with a log-scale color mapping" looks similar in x11 and Qt 
terminals.  In fact, shrinking down the image I can see in both cases 
that "NaN should appear as background" is actually in front of the 
pixmap while "Negative values become NaN with a log-scale color mapping".

I'm fine with the demo, but now that I understand it I don't see the 
terminal type as being highly responsible for the feature.  (Partial 
transparency is a different issue.  That's something more significant. 
Could probably do alpha-blending in X11, but I don't think anyone wants 
to tackle that.)  The "failsafe" option is somewhat vague.  Given what I 
suspect about the image being actually drawn with individual rectangles 
(something that "image" defaults to when cast into a 3D 
non-perpendicular view), it might have been nicer to force this mode 
with a different type, say "pixmap" as opposed to "image".

I'm not too quick to dismiss x11 terminal.  Yes, I know maintenance is 
an issue, and alpha blending is missing, antialiasing for nice lines is 
missing, etc.  But it wouldn't surprise me if ultimately Qt terminal 
(i.e., QGraphicsItem) is using X11 in some form.

Dan